Adaptive color space transform coding

ABSTRACT

An encoder system may include an analyzer that analyzes a current image area in an input video to select a transform. A selectable residue transformer, controlled by the analyzer, may perform the selectable transform on a residue image generated from the current image area and a predicted current image area, to generate a transformed residue image. An encoder may encode the transformed residue image to generate output data. The analyzer controls the encoder to encode information to identify the selectable transform and to indicate that the selectable transform for the current image area is different from a transform of a previous image area of the input video. A decoder system may include components appropriate for decoding the output data from the encoder system.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.13/905,889, filed on May 30, 2013, which is incorporated herein byreference in its entirety.

BACKGROUND

Image data, such as those contained in a video, may contain large amountof information relating to color, pixel location, and time. In order tohandle such large amount of information, it may be necessary to compressor encode the image data, without losing too much information from theoriginal video, while at the same time, without increasing thecomplexity of the data compression, which might decrease the speed ofimage data processing. Encoded image data may need to be decoded laterto convert back or restore the original video information.

To encode an image, pixel color data may be first transformed to colordata in an appropriate color-space coordinate system. Then, thetransformed data is encoded. For example, the image data may have rawpixel color data in a Red-Green-Blue (RGB) color space coordinatesystem. To encode the image data, the raw pixel color data in RGB colorspace may be transformed into color data in a YCbCr color spacecoordinate system, by separating the luminance component and the colorcomponent. Then, the color data in YCbCr color space coordinate systemmay be encoded. By doing so, redundant information that might existbetween the original three colors may be compressed by removing theredundancy during the color space transform.

Additional redundancies in the image data may be removed during theencoding of the transformed image data, by performing spatial predictionand temporal prediction, followed by additional encoding of anyremaining residual data to any extent that is desirable, as well as byentropy encoding of the data in an individual frame in a point in timeand/or of the data in a duration of the video sequence. Spatialprediction may predict image data in a single frame in time to eliminateredundant information between different pixels in the same frame.Temporal prediction may predict image data in a duration of the videosequence to eliminate redundant information between different frames. Aresidue image may be generated from the difference between thenon-encoded image data and the predicted image data.

Some color space formats, such as RGB 4:4:4, may be less efficient tocode natively since the different color planes may have not beeneffectively de-correlated. That is, redundant information may existbetween different components that may not be removed during encoding,resulting in a reduced coding efficiency versus an alternative colorspace. On the other hand, it may be undesirable to encode this materialin an alternative color space such as YUV 4:4:4 or YCoCg and YCoCg-R4:4:4 in some applications, because of the color transformation that mayhave to be performed outside the coding loop, as well as possible lossesthat may be introduced through the color transformation.

Thus, there is a need for an improved way of transforming and encodingimage data efficiently.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an encoding system according to an embodiment of thepresent disclosure.

FIG. 2 illustrates a decoding system according to an embodiment of thepresent disclosure.

FIG. 3 illustrates an encoding method according to an embodiment of thepresent disclosure.

FIG. 4 illustrates a decoding method according to an embodiment of thepresent disclosure.

DETAILED DESCRIPTION

According to an embodiment as illustrated in FIG. 1, a system 100 mayinclude an analyzer 130, a selectable residue transformer 160, and anencoder 170.

The analyzer 130 may analyze a current image area in an input video 110to select a transform. The selectable residue transformer 160 may becontrolled by the analyzer 130, to perform the selectable transform on aresidue image generated from the current image area and a predictedcurrent image area, to generate a transformed residue image. The encoder170 may encode the transformed residue image to generate output data190. The analyzer 130 may control the encoder 170 to encode informationto identify the selectable transform and to indicate that the selectabletransform for the current image area is different from a transform of aprevious image area of the input video.

Optionally, the system 100 may include frame buffers 120 to storeinformation of input video 110, for example, image data previouslyprocessed. Such data in frame buffers 120 may be used by interprediction 150, controlled by the analyzer 130, to perform temporalpredictions, i.e. generating predicted image data for the current imagearea based upon the data of a previous frame. Or such data in framebuffers 120 may be used by intra prediction 152, controlled by theanalyzer 130, to perform spatial predictions, i.e. generating predictedimage data for the current image area based upon the data of anotherportion of the current frame. Optionally, the analyzer 130 may performits analysis based upon the data stored in the frame buffers 120.Predicted image area for the current image area generated by the interprediction 150 and/or the intra prediction 152 may be combined with (orsubtracted from) the current image area of the input video 110 by anintegrator 140, to generate the residue image.

According to an embodiment, the current image area may be one of aframe, a slice, and a coding tree unit. The selectable transform mayinclude a color-space transform. The encoder 170 may include an entropyencoder. The encoded information identifying the selectable transformmay specify coefficients of a selectable inverse transform. The encodedinformation identifying the selectable transform may be contained in oneof a sequence parameter set, a picture parameter set, and a sliceheader, preceding encoded residue image data of the current image area.The encoder 170 may include a transformer 172 and/or a quantizer 174,which may be controlled by the analyzer 130 to perform quantization.

The analyzer 130 may select and change the selectable transform for theselectable residue transformer 160 and alter parameters accordingly, forexample for inter prediction 150, intra prediction 152, and encoder 170,to optimize for data encoding, data decoding, encoded data size, errorrate, and/or system resources required for encoding or decoding.

The next generation High Efficiency Video Coding (HEVC) standardintroduced several new video coding tools in an effort to improve videocoding efficiency versus previous video coding standards andtechnologies such as MPEG-2, MPEG-4 part2, MPEG-4 AVC/H.264, VC1, andVP8 among others.

The new standard may support the encoding of YUV 4:2:0 8 or 10 bitmaterial using well defined profiles, for example, the Main, Main 10,and Main Still Picture profiles. There is considerable interest inprofessional applications, such as cinema applications, capture, videoediting, archiving, gaming, and consumer applications, especially forscreen content compression and sharing, to develop formats to supporthigher (more than 10 bits) sample precision (bit-depth) as well asdifferent color sampling formats and color spaces, including YUV or RGB4:4:4.

Encoding principles of higher color sampling formats/spaces may besimilar to those of formats with less sampling precision, i.e. 4:2:0YUV, to appropriately handle the difference in resolution for the chromacomponents. One of the color components may be perceived as equivalentto the luma component in the 4:2:0 YUV encoding, whereas the remainingcolor components may be handled similarly as the chroma components,while accounting for the higher resolution. That is, prediction toolssuch as intra prediction and motion compensation, need to account forthe increment in resolution, and the transform and quantizationprocesses, also need to handle additional residual data for the colorcomponents. Similarly, other processes such as entropy coding,deblocking and the sample adaptive offset (SAO) among others, may needto be extended to process the increase in video data. Alternatively, allcolor components may be encoded separately as separate monochromeimages, with each color component taking the role of the lumainformation during the encoding or decoding processes.

To improve coding performance, an additional color space transformationmay be performed on the residual data that may result in betterde-correlation (less redundancy) between all color components. Theselectable color space transformation may be applied on dequantized(inverse quantized) and inverse-transformed residual data using anadaptively derived color-space transform matrix, such as:

${\begin{bmatrix}a & b & c \\d & e & f \\g & h & i\end{bmatrix}^{t}\begin{bmatrix}P^{\prime} \\Q^{\prime} \\R^{\prime}\end{bmatrix}} = \begin{bmatrix}G^{\prime} \\B^{\prime} \\R^{\prime}\end{bmatrix}$

The color transform matrix may be derived using previously restoredimage data, such as image data on the left or above of the currenttransform unit or image data of the transform unit in the previousframes. The derivation may involve normalizing the reference samples ineach color plane by subtracting their mean and by computing andnormalizing a covariance matrix across all color planes. This mayachieve some “localized” coding performance benefits, without adding anynew signaling overhead in the HEVC specification. However, this may addcomplexity in both encoder and decoder for the derivation of thetransformation parameters.

To simplify the adaptive color transform in video encoding and decoding,the color transforms are only applied on residual data. Additional colortransforms may be selectable and signaled by the encoder, and thedecoder may select and perform the corresponding inverse color transformbased upon the signaling decoded from the encoded data, according to thepresent invention.

In particular, one or more color transforms may be implicitly orexplicitly signaled at different levels within a codec such as HEVC. Forexample, an encoder may implicitly signal known color transforms, suchas transforms in the limited or full range YUV Rec.709, Rec.2020, orRec.601, as well as YCoCg, from the RGB color space. An encoder mayexplicitly signal color transforms, by signaling or specifying allinverse color transform coefficients with a predefined precision, forexample, by listing the transform coefficients or their relationships inportions of the encoded data. Color transforms, including the types, theparameters, and the coefficients, may be signaled or specified withinthe Sequence Parameter Set (SPS) NALU, the Picture Parameter Sets (PPS),and/or a Slice header. Signaling within a Coding Tree Unit (CTU) mayalso be possible, although that may cost a bit more in terms of bitrate,which may not be desirable.

If such transform information is specified for different levels of thevideo sequence, i.e. sequence, frame, and pixel block of a CTU, then thetransforms may be predicted within the hierarchy of these elements. Thatis, a transform in the PPS may be predicted from transforms defined inthe SPS, and transforms in the slice header may be predicted fromtransforms in the PPS and/or the SPS. New syntax elements and units maybe defined and used to allow this prediction of transforms betweendifferent levels of the video sequence hierarchy, to include theprediction or non-prediction of the transform from specified transformsor higher level transforms, as well as for the prediction ornon-prediction of the precision of the transform coefficients and thecoefficients themselves. Derivation of an explicitly defined colortransform may be based on data available, such as sample data from theentire sequence, picture, slice, or CTU. An encoder may choose or selectto use data that correspond to current pixel samples if available, oruse data from past frames or units that have already been encoded. Aprincipal component analysis method, e.g. a covariance method, iterativemethod, non-linear iterative partial least squares, etc., may be used toderive the transform coefficients.

A system may dictate that only a single transform shall be used for theentire sequence, thus not permitting any change of the color transformwithin any subcomponents of the sequence, i.e. within a picture, aslice, CTU, or a Transform Unit (TU), through signaling or semantics(i.e. forced by the codec or profile/level). A similar restriction maybe performed at a lower level, i.e. within a picture, slice, or CTU.

However, it may also be possible for a system to allow switching ofcolor transforms within a sequence, picture, slice, or even CTU.Switching of color transforms for every picture and slice may be done bysignaling new color transform parameters for each new data block thatoverride the higher level or previous block transform parameters.Additional transform parameters may be signaled at a lower layer,effectively allowing switching of the color transform for an entire CTU,Coding Unit (CU), or even TU. However such signaling may take up asignificant number of bits in the resulting encoded data stream, thusincreasing the data stream size.

Alternatively, the color transform may be derived based on a variety ofpredefined or signaled conditions in the bitstream. In particular, aparticular color transform may be pre-assigned to a specific transformblock size, coding unit size, or prediction mode (e.g. intra vs inter).For example, assuming that transform units of luma and chroma data arealigned for a particular video sequence, if the size of the lumatransform that is to be used is 16×16, then Color transform A is used,if 8×8 luma transform is to be used, color transform B is used, and for32×32 or 4×4 transforms no color transform is applied. If the transformunits of luma and chroma data are not aligned, then alternative butsimilar ways of pre-defining conditional derivation of color transformsmay be used to account for the misalignment of transform units.

A system may buffer or cache a number of pre-defined color transformsalong with associated processing algorithms, in encoding or decoding,such that the system may store a codebook that it can look up thepre-defined color transforms, for example via a lookup table (LUT). Thesystem may also compute or predict color transforms and store them inthe buffer for later lookup.

In some codec standards, prediction units (PUs) and TUs may be definedwithin a CU with no strict dependency between the two. Thus, predictionunits (PUs) and TUs may not be directly related in terms of size. Inother codec standards, if TUs are defined as strictly within PUs, thenPU information, such as prediction list and reference indices, may beused to derive the color transform.

For systems where complexity is not a concern, then a combination of theabove methods may be used. That is, for every CTU, CU, or transformblock, the encoder may signal in the encoded data stream whether to usea previously defined/signaled color transformed, or whether the colortransform should be derived separately for the current unit based onneighborhood information. This allows the system to control thecomplexity of the decoder and to avoid cases where there is insufficientinformation to derive the color transform from its neighbors. This maybe especially true around object or color boundaries or noisy data,where neighborhood data may be uncorrelated. The adaptively computedcolor transform may be computed and updated at less frequent intervals,e.g. every CTU row or even for every CTU, to reduce decoder complexity.Stability of the color transform may be increased by slowly adapting thecolor transform using previously generated values. That is, thecomputation of the current color transform at unit (e.g. Transform Unit)n, may be performed as:

Transform(n)=w ₀*Transform(n−1)+w ₁*ComputedTransform(n)

Where the ComputedTransform(n) is the transform that is estimated purelybased on local pixel group information. The two weights, w₀ and w₁, maybe predefined or signaled in the system providing further flexibility onhow to control the computation of the color transform. That is,increasing the value of w₀ relative to w₁, increases the dependence ofresulting color transform Transform(n) on a neighboring color transformTransform(n−1).

An encoding system may determine all of the transforms needed to encodea video sequence, by for example, analyzing the image data in the videosequence and perform cost-benefit evaluation to optimize the encoding,the decoding, quality of data, and/or size of encoded data. For example,if the encoding system has sufficient computational resources, then itmay perform a “brute force” analysis, by performing multiple possiblecolor transforms on all individual frames and transform units, and thenselect one color transform for each transform unit that results in theleast rate distortion, if rate distortion is to be optimized. However,such a “brute force” analysis would require a lot of computationalresources, and would be slow, and thus it may not be useful in anapplication where encoding needs to be done in near “real time”, forexample, in “live” video streaming.

The usage of different color transforms per block may impact otherportions of the encoding and decoding process. In particular, theentropy coding, e.g. based on the context adaptive binary arithmeticcoding (CABAC), assumes that coefficients in neighboring blocks are inthe same color domain, the statistics for the entropy coding process maybe accumulated accordingly, and deblocking may utilize the quantizationparameters (QP) used for each color component when filtering blockedges.

However, this may not be the case in a system that uses block leveladaptive color transforms which may impact coding performance. In thecase of entropy coding, the impact may be trivial and thus thedifference in color space may be ignored. Limiting the process inconsidering neighboring data that are in the same color space maypenalize performance in terms of complexity and implementation, becausemore contexts may need to be handled and compensated for each new colortransform that may have been used. Thus, the system may not need toalter the encoding process for the adaptive color transforms.

On the other hand, adaptive color transform changes may be easier toaccount for during deblocking. In particular, when deriving theappropriate thresholds for each color component for deblocking, thesignaled QP values may be used while ignoring the color space used, orthe QP values may be approximated in the native color domain given theQP values used for coding the transformed residual. For example, asimple way is to apply the same color transform that is applied on theresidual data to also the quantizer values, or define and signal anadditional transform that would help translate the used quantizer valuesfor the transformed residuals to native color space quantizer values.For simplicity, the system may not need to translate or adjustquantization values for the adaptive color transforms.

According to an embodiment as illustrated in FIG. 2, a system 200 mayinclude a decoder 230, a selectable residue inverse transformer 220, andan integrator 240.

The decoder 230 may receive and decode input data 210. The selectableresidue inverse transformer 220 may be controlled by the decoder 230, toperform a selectable inverse transform on the decoded input data, togenerate an inverse transformed residue image. The integrator 240 maycombine the inverse transformed residue image with a predicted image fora current image area to generate a restored current image area of anoutput video 290. The decoder 230 may select the selectable inversetransform based upon encoded information in the input data 210, theencoded information identifies the selectable inverse transform andindicates that the selectable inverse transform for the current imagearea is different from a transform of a previous image area of theoutput video 290.

Optionally, the system 200 may include frame buffers 280 to storeinformation of output video 290, for example, image data previouslyprocessed. Such data in frame buffers 280 may be used by interprediction 250, controlled by the decoder 230, to perform temporalpredictions, i.e. generating predicted image data for the current imagearea based upon the data of a previous frame. Intra prediction 260 maybe controlled by the decoder 230 to perform spatial predictions, i.e.generating predicted image data for the current image area based uponthe data of another portion of the current frame. Predicted image areafor the current image area generated by the inter prediction 250 and/orthe intra prediction 260 may be combined with (or added with) theinverse transformed residue image, from selectable residue inversetransformer 220, by the integrator 240, to generate the restored currentimage area of the output video 290. The system 220 may include anadjuster 270, which performs adjustments of restored current image areafor the output video 290. The adjuster 270 may include deblocking 272and sample adaptive offset (SAO) 274. The adjuster 270 may output to theoutput video 290 and/or the frame buffers 280.

According to an embodiment, the current image area may be one of aframe, a slice, and a coding tree unit. The selectable inverse transformmay include a color-space transform. The decoder 230 may include anentropy decoder. The encoded information identifying the selectableinverse transform may specify coefficients of a selectable inversetransform. The encoded information identifying the selectable inversetransform may be contained in one of a sequence parameter set, a pictureparameter set, and a slice header, preceding encoded residue image dataof the current image area. The decoder 230 may include an inversetransformer 232 and/or an inverse quantizer 234, which may performquantization. The output video 290 may be connected to a display device(not shown) and displayed.

The decoder 230 may select and change the selectable inverse transformfor the selectable residue inverse transformer 220 and alter parametersaccordingly, for example for inter prediction 250, intra prediction 260,and adjuster 270, based upon encoded information in the input datareceived identifying the selectable inverse transform.

FIG. 3 illustrates a method 300 according to an embodiment.

The method 300 may include block 310, analyzing, by an analyzer, acurrent image area in an input video to select a transform.

At block 320, performing, by a selectable residue transformer controlledby the analyzer, the selectable transform on a residue image generatedfrom the current image area and a predicted current image area, togenerate a transformed residue image.

At block 330, encoding, by an encoder, the transformed residue image togenerate output data.

According to an embodiment, the analyzer may control the encoder toencode information to identify the selectable transform and to indicatethat the selectable transform for the current image area is differentfrom a transform of a previous image area of the input video.

According to an embodiment, the analyzer may analyze the input video andselect an overall sequence color transform for the entire videosequence, and analyze and select residue color transforms for individualframes, slices, pixel blocks, CTUs, etc. The analyzer may continuallyanalyze the input video, and perform selecting of the color transformin-situ for each frame as the input video is received and encoding isprocessed. Or the analyzer may analyze the entire input video sequencecompletely before selecting the color transforms and beginning theencoding.

FIG. 4 illustrates a method 400 according to an embodiment.

The method 400 may include block 410, receiving and decoding, by adecoder, input data.

At block 420, performing, by a selectable residue inverse transformercontrolled by the decoder, a selectable inverse transform on the decodedinput data, to generate an inverse transformed residue image.

At block 430, combining, by an integrator, the inverse transformedresidue image with a predicted image for a current image area togenerate a restored current image area of an output video.

According to an embodiment, the decoder may select the selectableinverse transform based upon encoded information in the input data, theencoded information identifies the selectable inverse transform andindicates that the selectable inverse transform for the current imagearea is different from a transform of a previous image area of the inputvideo.

It is appreciated that the disclosure is not limited to the describedembodiments, and that any number of scenarios and embodiments in whichconflicting appointments exist may be resolved.

Although the disclosure has been described with reference to severalexemplary embodiments, it is understood that the words that have beenused are words of description and illustration, rather than words oflimitation. Changes may be made within the purview of the appendedclaims, as presently stated and as amended, without departing from thescope and spirit of the disclosure in its aspects. Although thedisclosure has been described with reference to particular means,materials and embodiments, the disclosure is not intended to be limitedto the particulars disclosed; rather the disclosure extends to allfunctionally equivalent structures, methods, and uses such as are withinthe scope of the appended claims.

While the computer-readable medium may be described as a single medium,the term “computer-readable medium” includes a single medium or multiplemedia, such as a centralized or distributed database, and/or associatedcaches and servers that store one or more sets of instructions. The term“computer-readable medium” shall also include any medium that is capableof storing, encoding or carrying a set of instructions for execution bya processor or that cause a computer system to perform any one or moreof the embodiments disclosed herein.

The computer-readable medium may comprise a non-transitorycomputer-readable medium or media and/or comprise a transitorycomputer-readable medium or media. In a particular non-limiting,exemplary embodiment, the computer-readable medium can include asolid-state memory such as a memory card or other package that housesone or more non-volatile read-only memories. Further, thecomputer-readable medium can be a random access memory or other volatilere-writable memory. Additionally, the computer-readable medium caninclude a magneto-optical or optical medium, such as a disk or tapes orother storage device to capture carrier wave signals such as a signalcommunicated over a transmission medium. Accordingly, the disclosure isconsidered to include any computer-readable medium or other equivalentsand successor media, in which data or instructions may be stored.

Although the present application describes specific embodiments whichmay be implemented as code segments in computer-readable media, it is tobe understood that dedicated hardware implementations, such asapplication specific integrated circuits, programmable logic arrays andother hardware devices, can be constructed to implement one or more ofthe embodiments described herein. Applications that may include thevarious embodiments set forth herein may broadly include a variety ofelectronic and computer systems. Accordingly, the present applicationmay encompass software, firmware, and hardware implementations, orcombinations thereof.

The present specification describes components and functions that may beimplemented in particular embodiments with reference to particularstandards and protocols, the disclosure is not limited to such standardsand protocols. Such standards are periodically superseded by faster ormore efficient equivalents having essentially the same functions.Accordingly, replacement standards and protocols having the same orsimilar functions are considered equivalents thereof.

The illustrations of the embodiments described herein are intended toprovide a general understanding of the various embodiments. Theillustrations are not intended to serve as a complete description of allof the elements and features of apparatus and systems that utilize thestructures or methods described herein. Many other embodiments may beapparent to those of skill in the art upon reviewing the disclosure.Other embodiments may be utilized and derived from the disclosure, suchthat structural and logical substitutions and changes may be madewithout departing from the scope of the disclosure. Additionally, theillustrations are merely representational and may not be drawn to scale.Certain proportions within the illustrations may be exaggerated, whileother proportions may be minimized. Accordingly, the disclosure and thefigures are to be regarded as illustrative rather than restrictive.

One or more embodiments of the disclosure may be referred to herein,individually and/or collectively, by the term “disclosure” merely forconvenience and without intending to voluntarily limit the scope of thisapplication to any particular disclosure or inventive concept. Moreover,although specific embodiments have been illustrated and describedherein, it should be appreciated that any subsequent arrangementdesigned to achieve the same or similar purpose may be substituted forthe specific embodiments shown. This disclosure is intended to cover anyand all subsequent adaptations or variations of various embodiments.Combinations of the above embodiments, and other embodiments notspecifically described herein, will be apparent to those of skill in theart upon reviewing the description.

In addition, in the foregoing Detailed Description, various features maybe grouped together or described in a single embodiment for the purposeof streamlining the disclosure. This disclosure is not to be interpretedas reflecting an intention that the claimed embodiments require morefeatures than are expressly recited in each claim. Rather, as thefollowing claims reflect, inventive subject matter may be directed toless than all of the features of any of the disclosed embodiments. Thus,the following claims are incorporated into the Detailed Description,with each claim standing on its own as defining separately claimedsubject matter.

The above disclosed subject matter is to be considered illustrative, andnot restrictive, and the appended claims are intended to cover all suchmodifications, enhancements, and other embodiments which fall within thetrue spirit and scope of the present disclosure. Thus, to the maximumextent allowed by law, the scope of the present disclosure is to bedetermined by the broadest permissible interpretation of the followingclaims and their equivalents, and shall not be restricted or limited bythe foregoing detailed description.

1-18. (canceled)
 19. A non-transitory computer readable mediumcomprising instructions that, when executed by a processor, cause:decoding encoded video data to determine transformed residual sampledata and to determine a selected inverse color transform for a currentimage area; performing the selected inverse color transform on thetransformed residual sample data to produce inverse transformed residualsample data; and combining the inverse transformed residual sample datawith motion predicted image data to generate restored image data for thecurrent image area of an output video, wherein the current image area isa transform unit (TU) and wherein the encoded video data identifying theselected inverse color transform is contained in a transform unitsyntax.
 20. The computer readable medium of claim 19, wherein a firstcolor component of the inverse transformed residual sample data isproduced based on only one color component of the transformed residualsample data, and the other color components of the inverse transformedresidual sample data are predicted based on the first color component ofthe inverse transformed residual sample data.
 21. The computer readablemedium of claim 19, wherein the encoded video data is split intoindividual color components, and wherein processing paths for each colorcomponent are separate from each other.
 22. The computer readable mediumof claim 19, wherein the decoding encoded video data includes entropydecoding.
 23. The computer readable medium of claim 19, wherein theencoded video data identifying the selected inverse color transformspecifies two coefficients of the selected inverse color transform.